WiremanWireman · Megger a cable or motor: troubleshooting for electricians · 2023 NEC ·
Megger a cable or motor
Isolate it, pick the test voltage, run the insulation test, read the result and discharge.
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How it works
An insulation tester (megger) puts a high DC voltage, like 500 or 1000 V, on a conductor and measures the tiny current that leaks through the insulation to ground or to other conductors. The result is in megohms. A multimeter’s 9 V can’t find weak insulation; a megger stresses it near working conditions and shows insulation that’s getting wet, dirty or damaged before it fails.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What are you testing?
- A cable, feeder or branch circuitTest: Isolate the cable
- Yes, fully isolatedTest: Run the insulation testExpect: High and steady or rising. New, dry cable often reads hundreds or thousands of megohms. For motors under 1000 V, IEEE 43 gives 5 MΩ as a minimum; healthy motors usually read far higher.
- High and steady or risingInsulation is good
- Low, but well above zeroTest: Run a polarization index testExpect: A PI of 2 or more generally means clean, dry insulation (IEEE 43 guidance for most modern insulation).
- PI of 2 or moreLow reading but healthy-acting insulation
- PI under about 2, flat readingsMoisture or contamination
- Near 0, or it suddenly dropsInsulation failure
- No, something can’t be disconnectedDon’t megger until it’s isolated
- Yes, fully isolatedTest: Run the insulation testExpect: High and steady or rising. New, dry cable often reads hundreds or thousands of megohms. For motors under 1000 V, IEEE 43 gives 5 MΩ as a minimum; healthy motors usually read far higher.
- A motor or other windingTest: Isolate the motor
- Yes, leads disconnectedTest: Run the insulation testExpect: High and steady or rising. New, dry cable often reads hundreds or thousands of megohms. For motors under 1000 V, IEEE 43 gives 5 MΩ as a minimum; healthy motors usually read far higher.
- High and steady or risingInsulation is good
- Low, but well above zeroTest: Run a polarization index testExpect: A PI of 2 or more generally means clean, dry insulation (IEEE 43 guidance for most modern insulation).
- PI of 2 or moreLow reading but healthy-acting insulation
- PI under about 2, flat readingsMoisture or contamination
- Near 0, or it suddenly dropsInsulation failure
- No, it can’t be separated from the driveDon’t megger until it’s isolated
- Yes, leads disconnectedTest: Run the insulation testExpect: High and steady or rising. New, dry cable often reads hundreds or thousands of megohms. For motors under 1000 V, IEEE 43 gives 5 MΩ as a minimum; healthy motors usually read far higher.
- A cable, feeder or branch circuitTest: Isolate the cable